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Inter-Process Communication Inter-Process Communication Lecture 5 Disclaimer: some slides are adopted from the book authors’ slides with permission 1 Recap • What are the three components of a process? – Address space – CPU context – OS resources • What are the steps of a context switching? – Save & restore CPU context – Change address space and other info in the PCB • What is the ready queue? – A list (or tree) of ready processes 2 Recap int count = 0; • Hints int main() { – Each process has its own int pid = fork(); private address space if (pid == 0){ count++; – Wait() blocks until the printf("Child: %d\n", count); child finish } else{ wait(NULL); count++; • printf("Parent: %d\n", count); Output? } Child: 1 count++; printf("Main: %d\n", count); Main: 2 return 0; Parent: 1 } Main: 2 3 Inter-Process Communication (IPC) • What is it? – Communication among processes • Why needed? – Information sharing – Modularity – Speedup 4 Chrome Browser • Multi-process architecture • Each tab is a separate process – Why? – How to communicate among the processes? 5 Models of IPC message passing shared memory 6 Models of IPC Shared memory share a region of memory between co-operating processes read or write to the shared memory region ++ fast communication -- synchronization is very difficult Message passing exchange messages (send and receive) typically involves data copies (to/from buffer) ++ synchronization is easier -- slower communication 7 Interprocess Communication in Unix (Linux) • Pipe • FIFO • Shared memory • Socket • Message queue • … 8 Pipes Most basic form of IPC on all Unix systems Your shell uses this a lot (and your 1st programming project too) ls | more Characteristics Unix pipes only allow unidirectional communication Communication between parent-child Processes must be in the same OS Pipes exist only until the processes exist Data can only be collected in FIFO order 9 IPC Example Using Pipes main() { char *s, buf[1024]; int fds[2]; s = “Hello World\n"; /* create a pipe */ (*) Img. source: http://beej.us/guide/bgipc/output/html/multipage/pipes.html pipe(fds); /* create a new process using fork */ if (fork() == 0) { /* child process. All file descriptors, including pipe are inherited, and copied.*/ write(fds[1], s, strlen(s)); exit(0); } /* parent process */ read(fds[0], buf, strlen(s)); write(1, buf, strlen(s)); } 10 Pipes Used in Unix Shells Pipes commonly used in most Unix shells output of one command is input to the next command example: ls| more How does the shell realize this command? create a pipe create a process to run ls create a process to run more the standard output of the process to run ls is redirected to a pipe streaming to the process to run more the standard input of the process to run more is redirected to be the pipe from the process running ls 11 FIFO (Named Pipes) Pipe with a name ! More powerful than anonymous pipes no parent-sibling relationship required allow bidirectional communication FIFOs exists even after creating process is terminated Characteristics of FIFOs appear as typical files communicating process must reside on the same machine 12 Example: Producer main() { char str[MAX_LENGTH]; int num, fd; mkfifo(FIFO_NAME, 0666); // create FIFO file fd = open(FIFO_NAME, O_WRONLY); // open FIFO for writing printf("Enter text to write in the FIFO file: "); fgets(str, MAX_LENGTH, stdin); while(!(feof(stdin))){ if ((num = write(fd, str, strlen(str))) == -1) perror("write"); else printf("producer: wrote %d bytes\n", num); fgets(str, MAX_LENGTH, stdin); } } 13 Example: Consumer main() { char str[MAX_LENGTH]; int num, fd; mkfifo(FIFO_NAME, 0666); // make fifo, if not already present fd = open(FIFO_NAME, O_RDONLY); // open fifo for reading do{ if((num = read(fd, str, MAX_LENGTH)) == -1) perror("read"); else{ str[num] = '\0'; printf("consumer: read %d bytes\n", num); printf("%s", str); } }while(num > 0); } 14 Shared Memory Process A’s Virtual memory Physical memory Process B’s Virtual memory 15 Shared Memory • Kernel is not involved in data transfer – No need to copy data to/from the kernel • Very fast IPC – Pipes, in contrast, need to • Send: copy from user to kernel • Recv: copy from kernel to user – BUT, you have to synchronize • Will discuss in the next week 16 POSIX Shared Memory • Sharing between unrelated processes • APIs – shm_open() • Open or create a shared memory object – ftruncate() • Set the size of a shared memory object – mmap() • Map the shared memory object into the caller’s address space 17 Example: Producer $ ./writer /shm-name “Hello” int main(int argc, char *argv[]) { char str[MAX_LENGTH]; int fd; size_t len; fd = shm_open(argv[1], O_CREAT | O_RDWR, S_IRWXU | S_IRWXG); len = strlen(argv[2]); ftruncate(fd, len); addr = mmap(NULL, len, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0); close(fd); memcpy(addr, argv[2], len); return 0; } http://www.ittc.ku.edu/~heechul/courses/eecs678/F15/shm-writer.c 18 Example: Consumer $ ./reader /shm-name int main(int argc, char *argv[]) { char *addr; int fd; struct stat sb; fd = shm_open(argv[1], O_RDWR, 0); fstat(fd, &sb); addr = mmap(NULL, sb.st_size, PROT_READ, MAP_SHARED, fd, 0); close(fd); printf(“%s\n”, addr); return 0; } http://www.ittc.ku.edu/~heechul/courses/eecs678/F15/shm-reader.c 19 Sockets • Sockets – two-way communication pipe – Backbone of your internet services • Unix Domain Sockets – communication between processes on the same Unix system – special file in the file system • Client/Server – client sending requests for information, processing – server waiting for user requests • Socket communication modes – connection-based, TCP – connection-less, UDP 20 Example: Server int main(int argc, char *argv[]) { int listenfd = 0, connfd = 0; struct sockaddr_in serv_addr; char sendBuff[1025]; time_t ticks; listenfd = socket(AF_INET, SOCK_STREAM, 0); memset(&serv_addr, '0', sizeof(serv_addr)); memset(sendBuff, '0', sizeof(sendBuff)); serv_addr.sin_family = AF_INET; serv_addr.sin_addr.s_addr = htonl(INADDR_ANY); serv_addr.sin_port = htons(5000); bind(listenfd, (struct sockaddr*)&serv_addr, sizeof(serv_addr)); listen(listenfd, 10); while(1) { connfd = accept(listenfd, (struct sockaddr*)NULL, NULL); snprintf(sendBuff, “Hello. I’m your server.”); write(connfd, sendBuff, strlen(sendBuff)); close(connfd); } } 21 Example: Client int main(int argc, char *argv[]) { int sockfd = 0, n = 0; char recvBuff[1024]; struct sockaddr_in serv_addr; sockfd = socket(AF_INET, SOCK_STREAM, 0); memset(&serv_addr, '0', sizeof(serv_addr)); serv_addr.sin_family = AF_INET; serv_addr.sin_port = htons(5000); inet_pton(AF_INET, argv[1], &serv_addr.sin_addr); connect(sockfd, (struct sockaddr *)&serv_addr, sizeof(serv_addr)); while ( (n = read(sockfd, recvBuff, sizeof(recvBuff)-1)) > 0) { recvBuff[n] = 0; printf("%s\n" recvBuff); } return 0; } $ ./client 127.0.0.1 Hello. I’m your server. 22 Remote Procedure Calls Remote procedure call (RPC) abstracts subroutine calls between processes on networked systems subroutine executes in another address space uses message passing communication model messages are well-structured RPC daemon on the server handles the remote calls Client-side stub proxy for the actual procedure on the server responsible for locating correct port on the server responsible for marshalling the procedure parameters Server-side stub receives the message unpacks the marshalled parameters performs the procedure on the server, returns result 23 Marshalling Parameters 24 Execution of RPC 25 Quiz • A process produces 100MB data in memory. You want to share the data with two other processes so that each of which can access half the data (50MB each). What IPC mechanism will you use and why? 26 .
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